This invention relates to switching apparatus for monitoring changes in signal status, i.e., changes in polarity, occurring on signaling lines of a telecommunications system.
To evaluate the switching condition of signaling wires and of subscriber lines and interexchange trunks, it is a common technique to employ relays which, in response to changes of the signaling condition of the particular line in question, change their states of excitation and contact positions. The relays must now, however, pick up in response to the occurrence of permissible interference voltages or shunt currents. Such relays are difficult to use, however, because they cannot be designed to be suitable for every operating condition. The uncertainty in using relays for this purpose is at its greatest when large interference voltages and line resistances are encountered. The same is true with the conventional use of magnetic cores, the control windings of which are affected by the current to be measured.
In another arrangement of known construction the evaluation means is connected between the resistances of a voltage divider comprising two resistors and forming a bridge circuit. This produces independence from noise and supply voltage variations.
In another circuit for supervising a subscriber line, both signaling wires are routed through summing resistances to the input of a monitoring amplifier, one wire is connected directly to the amplifier, and the other wire is connected by way of a phase inverter stage. This arrangement is so designed that if currents flow in both wires that are equivalent in magnitude and direction, mutual compensation of the corresponding potential variations occurs at the input of the amplifier. If a counter-current flow is produced through the closed subscriber loop, a potential shift occurs at the input of the monitoring amplifier, and this then produces an appropriate signaling.
To determine the status of a subscriber line, it is also common practice to use a differential amplifier connected having inputs end connected to the taps of two voltage dividers which are linked and which are in turn connected to the line being supervised. This amplifier generates an output signal indicating the status of the supervised line as a function of the potential difference between the taps of these voltage dividers. By means of a bias resistor, it is ensured that the potential difference between these taps corresponds to a prespecified minimum value. The circuit is designed such that in response to a change of the switching condition of the subscriber line the relative polarities at these circuit points intended for the tap undergo a reversal.
It is, therefore, an object of the invention to provide a line monitoring means which easily adapts to variable conditions, and at the same time, is capable of acting to prevent the influence of noise components during the evaluation, to extend maximum protection against destruction by atmospheric discharges and shock-hazard voltages and to perform these functions with the greatest possible reliability.